This invention is concerned with improving the elevated temperature strength of Ni-30 Cu alloys without affecting their oxidation characteristics. The invention is particularly directed to improving these alloys for use in catalytic reactors.
Nickel base alloys containing about 30% copper are used at moderate temperatures where good corrosion resistance is required. Alloys known commercially as Monels, a registered trademark of the International Nickel Company, have been satisfactory for such uses. U.S. Pat. No. 3,842,159 to Niebylski et al discusses the application of Monel metal to automotive NO.sub.x emissions control.
Monel alloy 400 has been of particular interest as a catalyst for the reduction of nitrogen oxide. However, this alloy lacks long term durability. This characteristic appears to be related to grain boundary degradation and subsequent loss in strength.
Oxidation behavior of the Monel alloy is important in catalytic applications because the formation of NiO and CuO oxides on the Monel surface play an active role in the reduction of NO.sub.x. Thus, any attempt to strengthen the Ni-30 Cu base alloys must not affect the overall surface oxidation characteristic. For example, the use of the high strength aluminum-modified Monel alloy K-500 is undesirable because a continuous unreducible alumina scale could be formed.
Various other alloying materials have been added to nickel-copper alloys to alter the characteristics of the alloys. For example, Fahrenwald U.S. Pat. No. 1,346,190 teaches increasing the hardness and mechanical strength of gun barrels by adding various metals which are soluble in one or the other primary ingredient of a nickel-copper alloy. The strength of nickel-copper cast valves at elevated temperatures has been improved by the addition of zirconium, as set forth in Wise et al. U.S. Pat. No. 2,283,246. However, the prior art teachings have not been directed to improving the high temperature mechanical properties without affecting the oxidation characteristics.